Dissection of mature human T cells into those that mediate predominantly helper/inducer effects (Leu 3, T4) and those that mediate predominantly suppressor/cytotoxic effects (Leu 2, T8) was made possible with monoclonal antibodies. Using the mixed leukocyte reaction (MLR) as a model system, we have further fractionated these two major T lineages into subsets with relatively narrow functional repetoires. For example, immunoglobulin synthesis induced in MLR was shown to be regulated by sequential interactions between subsets of Leu 2-,3+ suppressor inducer cells, Leu 2+,3-DR+ suppressor-amplifier cells and Leu 2+,3-,DR- suppressor-effector cells. We further showed that Leu 3+ cells capable of helping B cell differentiation and Ig synthesis can be distinguished from Leu 3+ cells that do not provide help on the basis of their expression of the Leu 8 marker. Finally, precursors of Leu 2+ suppressor-effector cells (Ts) were distinguished from precursors of Leu 2+ cytotoxic cells on the basis of their differential expression of the 9.3 antigen. In summary, these data indicate that human T lymphocytes consist of a larger number of functionally distinct subsets than heretofore recognized, distinguishable from one another with combinations of monoclonal antibodies directed at cell surface markers.